Several caralysts having activity for the catalytic conversion of alcohols into ethers are known, the so called acidic dehydration catalysts. As examples of known acidic dehydration catalysts alumina, e.g. .gamma.-alumina, silica, alumina-silica mixtures and crystalline aluminosilicates, e.g. zeolites and smectites may be mentioned. A common feature of the majority of known acidic dehydration catalysts having an activity sufficiently high for industrial use is a short life because of rapid deactivation. Another drawback of the majority of known acidic dehydration catalysts is that at the same time they catalyze side reactions, resulting in an undesired high content of by-products in the ethers prepared.
The preparation of ethers from alcohols is carried out at elevated pressure and elevated temperature in the presence of the said acidic dehydration catalysts and it proceeds according to the general reaction scheme: EQU R.sup.1 OH+R.sup.2 OH.fwdarw.R.sup.1 OR.sup.2 +H.sub.2 O (1)
wherein R.sup.1 and R.sup.2 may be like or different and each denote optionally substituted alkyl, aryl or aralkyl groups. However, at the same time as the above desired reaction (1) also undesired side reactions occur, causing, i.a., the formation of hydrocarbons and deactivation of the catalyst employed by depositing so called coke consisting of polymerization products and/or carbon. A content of hydrocarbons in the ethers prepared in itself is undesired. The deactivation of the catalyst in consequence of deposition of coke is however much more serious because it is an accelerating process which rapidly requires the catalyst to be replaced or regenerated. Extensive research has been made especially in order to find catalysts having higher selectivity for the ether formation and less tendency to coke-formation, and moreover to determine optimum process conditions. So far as the crystalline aluminosilicates are concerned, for example, numerous attempts have been made at bringing about improved properties by varying the pore structure and/or by causing atoms of other elements to form part of the crystal lattice. However, hitherto these attempts have been without much success.
Earlier, numerous experiments have been carried out involving the treatment of various catalysts, i.a. crystalline aluminosilicates, with various reagents, i.a. various nitrogen-containing bases. In these experiments it has been demonstrated that the nitrogenous bases become bonded to (poison) the acidic active sites of the catalysts and thereby extensively block the catalytic activity thereof. It is thus known that treatment of crystalline aluminosilicates with nitrogen-containing bases cause these catalysts, which in the untreated state possess catalytic activity for, i.a., the above reaction (1) to, no longer have this activity or only have it to a low degree. Moreover, it has been demonstrated that the blocking of the active sites with nitrogen-containing bases proceed to a high degree of completeness. The treatment of catalysts containing acidic active sites with for example ammonia therefore has also been utilized in the analytical chemistry because the amount of for example ammonia absorbed permits a rather exact determination of the number of acidic active sites of the catalyst in question (N. Tops e, K. Pedersen, E. G. Derouane, J. Catal. 70, 41-52 (1981)).
It has now been found that by a special treatment of crystalline aluminosilicates known as catalysts it is possible to impart to these a lower deactivation rate and hence a longer life, and at the same time by using such catalysts to obtain a reaction product having a lower content of by-products.
More specifically, it has surprisingly been found that it is possible to effect a partial blocking of the active sites in crystalline aluminosilicates with nitrogen-containing bases whereby there is obtained a selective blocking of various acidic active sites in the catalyst. Thus, mainly a blocking of those active sites that are responsible for the formation of hydrocarbons takes place, whereas the active sites that catalyse ether formation according to the above reaction (1) are blocked to a far lesser extent.